An axial turbomachine has a casing and a rotor which is enclosed by the casing. The rotor has a hub contour which together with the inner contour of the casing forms a flow passage through the axial turbomachine. The rotor has a multiplicity of rotor stages which are formed in each case by a rotor blade cascade. The rotor blade cascades have a multiplicity of rotor blades which by one of their ends are fastened in each case on the rotor on the hub side and by their other end point radially outwards. A blade tip, which faces the inner side of the casing and is arranged directly adjacent thereto, is formed at this other end of the rotor blade. The distance between each blade tip and the inner side of the casing is formed as a radial gap which is dimensioned in such a way that on the one hand the blade tips do not rub against the casing during operation of the axial turbomachine and on the other hand the leakage flow through the radial gap, which ensues during operation of the axial turbomachine, is as low as possible. So that the axial turbomachine has high efficiency, it is desirable that the leakage flow through the radial gap is as low as possible.
If the axial turbomachine is installed in an aero engine, the casing is of a filigree construction in order to have a weight which is as low as possible. On the other hand, the rotor is solidly constructed in order to be able to withstand the pressure stresses and temperature stresses during operation of the axial turbomachine. The rotor blades are less solidly constructed in comparison to the rotor and are mounted on the rotor.
During operation of the axial turbomachine, the inner side of the casing and the rotor blades are in contact with hot gas, the casing having extensive contact with the hot gas on its inner side. Due to the fact that the casing is of a more filigree design than the rotor, the rotor heats up more slowly than the casing. This has the result that for startup and shutdown of the axial turbomachine the rotor and the casing have different rates of thermal expansion so that during startup and shutdown of the axial turbomachine the height of the radial gap, which is formed between the blade tips of the rotor blades and the inner side of the casing, changes. In this case, the radial gap is large during startup and small during shutdown. So that during shutdown the blade tips of the rotor blades do not butt against the casing and become damaged, the radial gap is provided with a minimum height which is dimensioned in such a way that during shutdown of the axial turbomachine the blade tips seldom, if ever, come into contact with the casing. This has the result that provision is made for a correspondingly dimensioned radial gap at the blade tips. On the other hand, especially during startup of the axial turbomachine, the radial gap is to be formed only large enough for a reduction of the power density and the efficiency of the axial turbomachine, brought about by the leakage flow, to be kept within acceptable limits.
Modern rotor blades have a very high aerodynamic efficiency which is achieved as a result of a high pressure load of the rotor blades. Brought about by this high pressure load, the leakage flow through the radial gap is particularly high so that as a result of the leakage flow the overall efficiency of the rotor blade is seriously impaired. Particularly in the case of rotor blades with small overall height and large radial gaps, about 50% of the overall loss of the rotor blades is caused by the leakage flow. A reduction of the leakage flow brings about an improvement of the overall efficiency of the rotor blade.
It is customarily known to reduce the leakage flow for example by means of an “active-clearance control” device. With the “active-clearance control” device, the casing is cooled during startup and heated up during shutdown so that the rate of thermal expansion of the casing is adapted to that of the rotor blades. Furthermore, for reducing the leakage flow a special profiling of the blade tips, such as the forming of a knife-blade-like blade tip, is known from U.S. Pat. No. 4,738,586.
A further blade tip, which is contoured in the direction of the span of the rotor blade, is known from EP 675 290 A2. The blade tip and the oppositely disposed passage wall are contoured corresponding to each other, the passage wall having an encompassing recess and the blade tip having a radial tip extension conforming to the recess. As a result of this measure, a quick reduction of the gas velocity in the region of the recess can be achieved, as a result of which the strength of shock waves is weakened.
A further blade-tip contouring and passage-wall contouring is gathered from FR 996967.